One of the more difficult problems associated with any borehole is to communicate measured data between one or more locations down a borehole and the surface, or between downhole locations themselves. For example, in the oil and gas industry it is desirable to communicate data generated downhole to the surface during operations such as drilling, perforating, fracturing, and drill stem or well testing; and during production operations such as reservoir evaluation testing, pressure and temperature monitoring. Communication is also desired to transmit intelligence from the surface to downhole tools, equipment, or instruments to effect, control or modify operations or parameters.
Accurate and reliable downhole communication is particularly important when complex data comprising a set of measurements or instructions is to be communicated, i.e., when more than a single measurement or a simple trigger signal has to be communicated. For the transmission of complex data it is often desirable to communicate encoded digital signals.
One approach which has been widely considered for borehole communication is to use a direct wire connection between the surface and the downhole location(s). Communication then can be made via electrical signal through the wire. While much effort has been spent on “wireline” communication, its inherent high telemetry rate is not always needed and its energy requirements cannot always be feasibly met.
Wireless communication systems have also been developed for purposes of communicating data between a downhole tool and the surface of the well. These techniques include, for example, communicating commands downhole via (1) electromagnetic waves; (2) pressure or fluid pulses; and (3) acoustic communication. Conventional sonic sources and sensors used in downhole tools are described in U.S. Pat. Nos. 6,466,513, 5,852,587, 5,886,303, 5,796,677, 5,469,736, 6,084,826, 6,466,513, 7,339,494, and 7,460,435.
It is useful for the wireless modems to know various data regarding the other wireless modems so that such wireless modems can efficiently communicate. For example, knowledge of the nearest neighbor in a testing pipe string is useful to be energy efficient and to find the shortest path between the surface and the downhole tools, with less hops. In fact, the network stabilization is quicker and easier. In the past, wireless modems have been programmed or otherwise adapted to communicate with a known neighboring wireless modem before such wireless modems are installed on a testing pipe string. However, a potentially major problem can arise where a network of wireless modems are programmed to communicate with a known neighboring wireless modem, and where the field engineers assemble the tool string with the network of wireless modems in an improper order/arrangement. In such situation, a communication signal could be lost between hops, preventing data and control signals from transmitting between the surface and a location downhole. As such, there is a need for a new and improved method for finding the identity, position or relative order of wireless modems within a network of wireless modems coupled to a communication channel such as a testing/drill/tubing string. With such a network discovery algorithm, a field engineer does not have to rely on a perfect order of placement for each wireless modem so that the wireless modems will know the identity of their nearest neighbors and thereby ensure a reliable network of communication.
In network industries operating above the surface of the Earth, flooding algorithms are used to discover the neighboring wireless modems. Flood algorithms work very well, however, it is known that they require many exchanges of messages making flood algorithms impractical in a downhole environment where power consumption is important and data rates are much slower.
Algorithms for determining a bit rate and/or a frequency for acoustic communication between two wireless modems have been proposed. See for example WO 2010/069633.
However, despite the efforts of the prior art, there exists a need for a wireless modem that can determine a path for communication between two or more wireless modems in a network communication system in a manner that is suitable for use in a downhole environment.